This chapter functions as an addendum on the contributions of Irish philosopher George Berkeley in the field of the philosophy of physical science. It summarizes Berkeley’s theses and terminology into 21 points, done in no particular order. Popper believes that, while flawed, Berkeley’s work on idealism and essentialism should be praised for having served as a protest against essentialist explanations in the field of science. Experts, including Immanuel Kant, have taken Newton’s theories for absolute fact, even though Newton himself did not believe that his theory was the ultimate explanation in the realm of physics. (This is indeed correct, since Einstein will later prove the incompleteness of Newton’s theories). Essentialism, Popper argues, is dogmatic and deters future progress.

Popper disagrees with Berkeley’s instrumentalism and positivism, which presents science as a mere tool for humans to achieve their goals, with no ability or responsibility to discuss the true nature of things. Popper instead proposes a third view, which is that science should aim to produce true theories, even if there is no absolute way of determining ultimate truth. This is because every subsequent theory is more rigorous than the last, and the search for increasingly complex theories can only lead to better approximations of what is true.